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 The Beauty of Central Auriga - Another Samyang 135mm Widefield The image above was captured with a Samyang 135mm F/2 and a Canon T3i. An interesting thing I noticed is that, when zoomed in a bit, you can see the stars begin to collectively form a circular shape around the center of the frame (which is right in-between IC405 and IC410) via the shape of the stars' coma relative to the stars' position in the frame. It's really not too noticeable when from far away but even being slightly zoomed in you can tell there's a lot of coma happening that all seem to circle around the center of the frame. Mainly with the brighter stars, but even with some smaller, medium sized stars. However, through individual subframes, you can't really notice it at all. But once stacked, it is quite noticeable and a bit annoying. Makes it look that much less like a great photo. Should I return and exchange my lens? Or is this fair game for a good copy of the Samyang 135? |
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Is this wide open? I'm not sure I would have the patience required to wait for a perfect Samyang lens at F/2. It's a good lens, I use it myself, but it's still a cheap lens. I might be wrong, if so I hope others correct me, but I would lower my expectations. I would consider a few solutions: - Stop it down to capture stars (do star removal on the F/2 stack). - Stop it down to F/2.2-2.8 and use it there if stars look good. - Separate the stars in post anyway and don't stretch them as far as you have done in this image. Over-stretching the stars will make it a lot less noticeable. With my dedicated camera I can adjust backfocus enough to get good stars stopped down a tiny bit, but I use it at F/2 for narrowband data regardless and toss the stars. |
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Jan Erik Vallestad: Yup this is at wide open. I'm gonna try stopping down to about F/2.8 and see how bad it is there. Unfortunate though, as already I'm only collecting half the light at that point. I tried separating the stars out and combining them stretched a lot less, but unfortunately the coma was so bad that Starnet would leave all of the coma behind. Meaning even if I stretched the stars less and re-combined them, the coma would still be there as Starnet failed to remove them. But again I should maybe just adjust my expectations, or maybe even just bite the bullet and move on to something better, like a Redcat51 or any of the alternatives. F/2 is nice but not when your stars look like this. |
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| Since this is with an APS-C DSLR the stars should really look a lot better. The lens could be a lemon. My initial reaction was that this is a spacing issue, then I noticed you’re using a DSLR, not an astro camera. Typically DSLRs don’t manifest spacing issues because the lens is manufactured to fit them. |
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| (edit) Looks like spacing to me. |
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Since this is with an APS-C DSLR the stars should really look a lot better. The lens could be a lemon. My initial reaction was that this is a spacing issue, then I noticed you’re using a DSLR, not an astro camera. Typically DSLRs don’t manifest spacing issues because the lens is manufactured to fit them. I agree. I had a D5300 in the past and the accompanying Rokinon 135mm I had did not look nearly as bad. The coma in this one is with practically every medium sized star. I also dont think it's a spacing issue, as that just wouldn't make sense for a DSLR. I think I'm gonna exchange it. The idea of shooting at F/2.8, while it might solve these issues, just defeats the purpose of having F/2 in the first place. |
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Jon Main: Is a spacing issue possible with a DSLR and a specifically manufactured lens, though? |
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Carl Elgario: I see. The problem is that with DSLR's there aren't really any wiggle room to add space etc, as there is with dedicated cameras. You could very well get a better copy but it might take a lot of trial and failure to get there. It might be the next one or it might be the tenth one. You also risk getting something worse. Did you by any chance use starnet? I could give StarX a go to see if it would be worth while considering that tool in stead. In my experience it works a lot better than Starnet. Shooting wide open on fast systems is very demanding on both the optics and camera, even the slightest 0.Xmm deviation will be extremely noticeable. Could be tilted elements in the lens, could be the sensor or the flange. Could be a bit of everything really. You could try ASTAP and evaluate what it tells you, then take it back to the vendor. FYI: I haven't tried my lens on anything but dedicated cameras, so I can't really compare to what it looks like either. I can only say that F/2 is unusable to me as well for this purpose, I know people use it at F/2.2 and F/2.8 though. IMO a good sacrifice to get better stars in the end result. |
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Carl Elgario:Jon Main: Yes. This is especially true when using a mount adapter. If you aren't using an adapter I'd get a new copy of the lens. |
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Jon Main:Carl Elgario:Jon Main: The lens is directly connected to the DSLR body without any adapters, so I believe it may just be a lens issue in this case. I'm trying to find alternatives instead of just going for another Samyang 135 because the idea of constantly exchanging lenses sounds like a nightmare. The newly releasing Sharpstar 51EDPH sounds intriguing, but I have no experience with Sharpstar and their EDPH line. |
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Jan Erik Vallestad:Carl Elgario: That's the thing. I could keep playing the lottery and try my hand at getting a good copy of this lens, but that may well take an incredibly, incredibly long time. So I'm thinking of alternatives, such as the Sharpstar EDPH line, but I've no idea where to go in this case because anything $750 USD is really my budget here. And also, I know this lens can look fantastic when stopped down, but I just feel that defeats the purpose of the F/2 in the first place. At that point, I may as well get an actual refractor. And yes I did use Starnet. Starnet2 to be specific. You can try using StarX and see if that does anything. What file will you need for that? |
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Carl Elgario:Jon Main:Carl Elgario:Jon Main: The Sharpstar 51 has nearly the same light gathering ability as the Rokinon at f/2.4 (one half-stop), except with significantly less FOV. I would try the Rokinon at f/2.4 and see if you’re happy with that, unless you don’t care about the FOV loss. |
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Carl Elgario:Jon Main:Carl Elgario:Jon Main: I actually prefer the FOV loss, just because I would prefer looking deeper into objects rather than getting wider fields. Mainly, the increased "nebula:star" ratio, as at 135mm the stars simply overpower a lot of the nebulae I want to capture. |
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Carl Elgario: Just a master file really, from the stack. Only if you're comfortable in doing so though, I have no ill intentions at all - but would respect and understand if you had any second thoughts about sharing your data. |
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Carl Elgario:Carl Elgario:Jon Main:Carl Elgario:Jon Main: Well in that case a traditional refractor like the Sharpstar is for you. Other than the price, the singular advantage of lenses like the Rokinon is the wide FOV. |
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Jan Erik Vallestad:Carl Elgario: I really don't mind, especially because the data's not really that good anyway, thanks to the coma. Here's a Google Drive link to the master stack: https://drive.google.com/drive/folders/1b5AiYMzIGzCe-q8TXE2zx3xlWyvCw86T?usp=drive_link |
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Carl Elgario:Carl Elgario:Jon Main:Carl Elgario:Jon Main: I'm pretty new to this whole thing so I've never actually attached a DSLR like the Canon T3i to a traditional refractor that needed a flattener. I've done it with astrographs that can just directly connect, but never with an actual refractor with a field flattener. The Sharpstar 51EDPH, which is the one I'm eyeing, will be used with a 0.84x field flattener. How do I go about attaching the DSLR to this field flattener, along with the proper 55mm of back focus? |
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Carl Elgario:Jon Main:Carl Elgario:Jon Main: I use this lens all the time for astro and have never seen sagittal astigmatism like what you are seeing here. That said, the lens is less sensitive to backfocus than some other lenses. For example: I also use the Tamron 35mm f/1.4 Di USD and the Sigma Art 14mm f/1.8. Using the normal Canon EF to R adapter the Sigma works but the Tamron has aberrations. Using the EF to R adapter with drop in filter the Sigma can't even come to focus while the Tamron works perfectly. The Rokinon doesn't seem to care much. Given that you aren't using an adapter I'd blame the lens. That said, if you are seeing the same aberration with multiple copies of the lens then it's possible there's an issue with the camera. I recently had my Canon R5 serviced and it has some sensor tilt that was fixed while addressing an unrelated electrical issue. |
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| It's the lens and the fact it's for a Canon so less backfocus. I'd venture you try your hand at a Nikon version and with the Nikon to Canon adaptor you also have more wiggle room with adjusting tilt/backfocus. My copy is neraly perfect @ f/2 (https://www.astrobin.com/ijq903/?nc=collection&nce=7793) and I wouldn't dream about stopping it down (otherwise what's the point?). |
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andrea tasselli: That is a very interesting proposition I have not yet heard of. Maybe that's why the previous Nikon version I had worked so well. I will keep this in mind. And I could not agree more about NOT stopping it down. I feel that's the whole point of F/2, the sheer light gathering capability. Stopping it down would just defeat that purpose, and I'd rather go with a cheaper APO refractor in that case just so I can at least get a more detailed FOV. |
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Carl Elgario: I noticed that your image is drizzled x2, personally I have had better results with star removal on this lens with un-drizzled data. But I'll give it a go. After ImageSolving > SPCC and SXT with lage overlap this is what remains:  It's simply the residue of worst stars in the image, those with the most coma. Otherwise it cleaned it up pretty good. The image is not stretched here, only applied a STF that vastly overstretches the image. Some clone stamping later:  Quickly processed into non-linear phase here (I didn't remove the halo):  The stars were a bit tricky as they were quite weirdly coloured unless stretched a bit too far for my liking in this case, but I managed to tone them down again:  And then finally back into the image:  |
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Forgot to do some SCNR on it. Here: 1:1  1:4  Admittedly, the stars don't look too great now either, but if I didn't stretch them enough they got this weird washed out orange/yellow/pink color.. |
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Jan Erik Vallestad:Carl Elgario: I am genuinely speechless. Wow. That looks absolutely phenomenal. So basically in summary, putting aside things I already did: SXT clone stamp combine the starlesss + stars at different stretches I am genuinely impressed with what you were able to do. I have had issues with this lens and the stars overpowering the nebulosity, but in this case they complement each other perfectly. I might just have to buy myself SXT then, darn it. Starnet2 did a pathetic job in comparison. |
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Jan Erik Vallestad: This is without any BlurX though right? I bet BlurX could do a great enough job making the stars look better, especially at this stretch level. I may still get myself a new lens, just as a “why not,” but you single handedly saved me from potentially a massive headache of looking for a replacement lens/refractor. |
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Carl Elgario: That's right, no BlurXterminator. I find that it struggles a bit with stars like these, often thinking they are several etc. So I prefer not to use it in such cases. Other than that it's all handled in PI. You can get the files and or steps followed. I'm sure it could be handled differently/better but it's not a complete mess. But it proves that you can get good results, just add a few minutes of star data while stopped down to add after removing the ones from F/2. Glad to be of help. |
